The main way the Peace of God movement tried to reduce this violence involved getting members of the nobility, especially knights, to take an oath not to harm clerics or the unarmed. They had a lot of support, especially from more powerful individuals, including at least one Duke of Aquitaine.
The phrase “members of the nobility” is rather flexible here since this was a time when the definition of who actually counted as noble was changing and expanding. For all that the desire to curb the violence against those with fewer means to defend themselves was absolutely genuine, the bishops’ motives weren’t entirely altruistic. The primary targets of their initiative were the newly powerful castellans and knights, whom they regarded as disruptive.
As time went on, the movement gained momentum and recognition and the idea of the Truce of God was added, under which people agreed not to fight or kill anyone at first just on Sundays, later on Fridays, Saturdays, important feasts, and seasons like Advent and Lent. By the late 11th century, nearly every ecclesiastical council in France acknowledged its importance.
This doesn’t mean that the movement was terribly effective. Actually, in practical terms, it probably wasn’t. But the bishops’ efforts would eventually have an impact on how members of nobility thought of themselves and their place in society throughout the next few hundred years. A lot of these ideals would eventually show up in both concepts of chivalry and the great romances.
*Including the Council of Cleremont, which launched the First Crusade. So, uh... yeah. Nice job with that one.
Bouchard, Constance Brittain. Strong of Body, Brave & Noble: Chivalry & Society in Medieval France. Ithaca, NY: Cornell University Press, 1998.
Barber, Malcolm. The Two Cities: Medieval Europe 1050-1320. London: Routledge, 1993.